The present invention relates to automatic document feeders.
When copies each are to be obtained from a corresponding one of a plurality of sheet documents, an automatic document feeder (ADF) is used which feeds the documents automatically one at a time onto a contact glass plate of the copy machine, exposes the fed document and then discharges the exposed document to a document receiver. When a plurality of copies is to be obtained from each of a plurality of documents, a recirculating automatic document feeder (RADF) is used in which the exposed document at ADF is returned to the document feeder table and stacked on the previously stacked documents and such circulation of the document is repeated automatically a desired number of times. There are two types of document feeding methods performed by ADFs inclusive of RADFs, one being an upside-down feeding method in which documents are stacked in sequence of pages on the document feeder table with their images facing upwardly, turned upside down one at a time and fed to the contact glass table of the copy machine, starting with the lowest final page, and another being a normal feeding method in which documents are to set on the document feeder table with their images facing downwardly and fed one at a time, starting with the lowest first page. Some of the former type employing a sheet through mode are introduced in which a document is exposed by a fixed optical system while being moved in order to increase the number of copies per minutes (CPM). Among the latter methods, no one is found which employs a sheet through mode to expedite high speed copying.
FIG. 1 shows a document feeding method using one example of the ADFs of the upside-down feeding type. Documents are stacked on a document feeder table 101 provided above a contact glass plate 100 such that the images face upwardly, sequentially turned upside down one at a time, starting with the lowest one, and fed to the contact glass plate 100, hit at its leading end on a reference 102 provided at the front end of the contact glass plate 100 to correct a skew in the document. The document is then exposed by moving an optical system, or otherwise the document is moved in the direction opposite to the feeding direction and exposed by a fixed optical system. The exposed document is carried in the reverse direction, discharged from the inlet side of the contact glass plate 100, turned upside down and discharged onto a document still present on the document feeder table 101 via discharge rollers 103. In that case, since the next document cannot be inserted onto the contact glass plate 100 until the exposed document is completely discharged out of the glass plate 100, the time when the feeding of a document starts is restricted, the intervals at which documents are fed are large and CPM is not improved. If a document is intended to be moved in the feeding direction on the contact glass plate 100 in a sheet through mode and exposed, the leading end of the document cannot be hit on the reference 102, so that no skews in the document can be corrected and the copy quality is deteriorated. Some of the upside-down feeding type which provides copies in a sheet through mode cannot free a document under exposure and is likely to cause uneven feeding and hence a defective image.
The sheet through exposure has been considered to be difficult in an ADF of the normal feeding type. The reason for this is that if the exposure position is set at the inlet end Pl of the contact glass plate 100, as shown in FIG. 2, the trailing end of the document under exposure will be at the separating and feeding section and the document feeder table, so that it is difficult to maintain the moving speed of the document constant due to a load due to the separation of the document and the weight of the bundle of documents 104 itself and thus to provide a good image. Second, when the exposure position is set in the vicinity of the front end P2 of the contact glass plate 100, the distance between the exposure position and a sensor 7 which determines the timing with which a sheet of transfer paper is fed is large and it is difficult to cause the leading ends of the sheet of transfer paper and image to coincide.
In view of the above prior art AFDs' problems, it is an object of the present invention to provide an ADF of a normal feeding type ADF which has a relatively simple mechanism which is capable of correcting a skew in the document even in a sheet through mode, minimizes the intervals at which a document is fed irrespective of the document size, greatly improves CPM and improves an image obtained under sheet through exposure.